1. Field of the Invention
The invention relates to computer graphics, and more specifically to software and tools for computer-assisted animation.
2. Related Art
Animation has long been an alternative to live-action photography for making films. The look of animation and its ability to exceed the bounds of the physically possible have allowed an art to develop feature films widely regarded as classics. Computer-assisted animation has reduced the labor required to produce an animated feature film and led to a boom in animated films in the 1990s.
Traditional animated films tend to have fairly static backgrounds. Furthermore, the "camera" in the animated film cannot move as freely as the camera in a live-action film, because unrestrained camera movement would require repainting of the background at each frame, a prohibitive expense.
Computer-assisted animation opens up the possibility of making the backgrounds of animated films more dynamic. Various techniques could be employed for this purpose. One possibility would generate backgrounds by means of computer rendering of 3-D objects. The animator creates a mathematical description of the background objects being animated and the light sources illuminating them. A computer program generates from that mathematical description an image which closely resembles a photograph of those objects. The process of generating such an image is referred to as "3-D rendering" or simply "rendering." Rendering is in a sense a computer simulation of photography. Within this framework, animation is achieved by specifying the movements of mathematically-modelled objects, the light sources illuminating them, and the imaginary camera taking the simulated photographs, and then rendering a suitable number of animation frames. This manner of making animated films was employed, for example, in the feature film Toy Story.
Unfortunately, computer rendering is not very suitable for backgrounds. It is often difficult to model mathematically the objects typically found in backgrounds, such as trees. Furthermore, all computer rendered objects have a distinctive hard-surfaced look, familiar to anyone who has seen Toy Story, which may be perceived as undesirable. This hard-surfaced look arises in part from the fact that the colors of these objects' surfaces are modeled by means of what is called a "texture map," in which the computer program in effect uses a two-dimensional drawing, generated separately by an artist, and covers the surfaces with that drawing somewhat in the manner of wallpaper to determine the surfaces' colors.
A refinement of computer rendering which has been suggested is to have the artist be able to see instantaneously what the scene will look like as he or she modifies the drawing used for texture mapping. In this way, the artist can more easily tailor that drawing to achieve a desired effect in the final rendered image. The artist is in effect drawing color directly onto the 3-D surface. This refinement is unfortunately both expensive to achieve in terms of computational resources and not a natural way of painting for the artist.
Ideally, an artist would make a two-dimensional drawing of a 3-D scene, and then a computer program would generate views of that 3-D scene from different camera angles using the artist's two-dimensional drawing. The artist would in effect be painting in the familiar way, in two dimensions, and the 3-D effects would be generated by the computer. This is, strictly speaking, impossible, because changes in the camera position could bring into view portions of objects which are occluded in the view painted by the artist, and some additional artist input is consequently required to establish how those occluded areas should look. It would be nonetheless desirable to approximate the ideal as closely as possible.
It is therefore desirable to allow an artist to paint backgrounds of animated films in the familiar manner in which a two-dimensional background is painted (with a computer paint program), and then to have the computer generate automatically views of the 3-D scene which those backgrounds depict from other camera angles.
It is also desirable to have computer's generation of additional views require little additional input from the artist, while still giving the artist the necessary creative control over the final product. Furthermore, it is desirable to have the additional views retain the same painterly appearance as the original view drawn by the artist.